Lithium ion secondary battery, separator, battery pack and charging method

ABSTRACT

A lithium ion secondary battery including a positive electrode, a negative electrode, a separator and a nonaqueous electrolyte, where the separator essentially includes a porous sheet. The positive electrode active material and the negative electrode active material can be reversibly doped and dedoped such that, where Qp (mAh) is an electric charge necessary for causing total lithium contained in the positive electrode to be dedoped and Qn (mAh) is an electric charge necessary for causing lithium to fully dope the negative electrode, Qp&gt;Qn. Further, when the battery is charged at a charging current Ic (mA) in a range of 0.2 Qn/h&lt;Ic&lt;2 Qn/h, in a range of an electric charge for charging Qc (mAh) of 1&lt;Qc/Qn&lt;Qp/Qn, doping of the positive electrode by lithium is initiated by producing lithium particles on the negative electrode by charging of the battery until Qc&gt;Qp.

TECHNICAL FIELD

The present invention relates to a low-cost lithium ion secondarybattery with high safety during overcharge, to a separator employedtherein, to a battery pack and to electric/electronic devices providedwith the lithium ion secondary battery, and to a charging method for thelithium ion secondary battery.

BACKGROUND ART

With the recent popularity and increasing performance of portableelectronic devices there has been a commensurate demand for secondarybatteries with high energy density. This demand has been met by greateruse of lithium ion secondary batteries which employ a carbon materialthat can be electrochemically doped and dedoped with lithium as thenegative electrode active material and a lithium-containing transitionmetal oxide as the positive electrode active material.

This type of lithium ion secondary battery undergoes charge anddischarge by migration of lithium ions between the positive and negativeelectrodes, by which storage and release of electrical energy areaccomplished. The lithium ion secondary battery has a high energydensity, because it outputs an average voltage of approximately 3.7 Vwhich is about 3 times that of conventional secondary batteries, but,because aqueous electrolyte solutions cannot be used as for conventionalsecondary batteries, non-aqueous electrolyte solutions with sufficientoxidation-reduction resistance are used. For this reason, lithium ionsecondary batteries are often referred to as non-aqueous secondarybatteries.

Because non-aqueous secondary batteries use a flammable non-aqueouselectrolyte solution as the electrolyte solution, there is a risk ofcombustion and, therefore, great caution must be taken for safety whenthey are used. While numerous situations can result in exposure to riskof combustion, overcharging is particularly dangerous.

In order to prevent overcharging, existing non-aqueous secondarybatteries are charged with a constant current and constant voltage, andthe batteries are provided with a precise protection circuit (safetycircuit: IC+FET×2). Such protection circuits are costly and thus add tothe cost of non-aqueous secondary batteries.

When overcharging is prevented by a protection circuit, the protectioncircuit sometimes fails to operate properly, and therefore it cannot beconsidered substantially safe. Existing non-aqueous secondary batteriesinclude modifications such as the provision of a safety vent, PTCelement and the use of a separator with a heat fuse function (shut downfunction) in order to safely destroy the battery in the event ofovercharging if the protection circuit breaks during overcharging.However, even when such a means is provided, the safety duringovercharging cannot always be reliably guaranteed, depending on theovercharging conditions and, in fact, combustion accidents still occurwith non-aqueous secondary batteries.

Since safety measures against overcharging with non-aqueous secondarybatteries are therefore still inadequate from the standpoint of safetyand cost, a problem to be solved has remained, and various methods havebeen proposed to improve the problem.

One approach for improvement is aimed at destroying the battery in asafer manner when the protection circuit fails to function. Examples ofthis approach include addition of a compound that readily generates agas upon overcharging and causes rapid actuation of a safety vent, asproposed in Japanese Patent No. 2928779, Japanese Patent No. 3061759,Japanese Patent No. 3113652, Japanese Unexamined Patent Publication No.2000-306610 and elsewhere, addition of a compound that polymerizes uponovercharging, thus blocking the current, as proposed in Japanese PatentNo. 3061756, and addition of a compound with an endothermic effect inthe event of overcharging, as proposed in Japanese Unexamined PatentPublication No. 11-45740; some of these methods involving additives havebeen implemented and have improved the safety of non-aqueous secondarybatteries.

Another approach is aimed at ensuring safety while also achieving costsavings by removal of the protection circuit or simple-protection aswith a thermistor system. Examples of this approach include the use ofredox shuttle additives, as proposed in Japanese unexamined PatentPublication No. 6-338347, Japanese Unexamined Patent Publication No.2000-251932, Japanese Unexamined Patent Publication No. 2000-277147,Japanese Unexamined Patent Publication No. 2000-228215 and elsewhere.Redox shuttle additives cause an oxidation-reduction reaction betweenthe positive and negative electrodes upon overcharging, thus preventingovercharging by a mechanism which consumes the overcharge current. Somesuch additives have been implemented and have contributed to theimproved safety of non-aqueous secondary batteries, but they have notsucceeded in removing the protection circuit or simplifying it.

Japanese Unexamined Patent Application No. 2000-67917 proposes atechnique related to preventing overcharging by employing a gel-polymerelectrolyte film, and this suggests the possibility of eliminating orsimplifying the protection circuit. However, the technique requires afilm thickness of no less than 30 μm for the gel-polymer electrolytefilm, with 40 μm or greater needed to obtain an adequate effect and evengreater thicknesses in order to achieve a notable effect. This thicknessis not widely used, though, considering that the separator thickness inmost existing non-aqueous secondary batteries is 25 μm, and that thetrend is toward a smaller separator thickness as increasingly higherenergy density is pursued for batteries.

Japanese Unexamined Patent Application No. 2000-123824 also proposes atechnique for preventing overcharging that employs a gel-polymerelectrolyte, thus suggesting the possibility of eliminating orsimplifying the protection circuit. According to this technique,overcharging is prevented by using a polyether oligomer, but thetechnique is not widely used because of the resulting very poordischarge characteristics of the battery compared to existingnon-aqueous secondary batteries.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to solve the above-mentionedproblems of the prior art by providing a non-aqueous secondary batterywhich maintains practical battery characteristics while permittingelimination of the protection circuit or simple-protection thereof to aheat-sensitive switch system such as a thermistor and/or PTC element,thus increasing safety during overcharging and lowering cost compared toconventional non-aqueous secondary batteries.

In order to solve these problems, the invention provides a lithium ionsecondary battery comprising a positive electrode, a negative electrode,a separator and a non-aqueous electrolyte, wherein

-   -   1) the separator is composed essentially of a porous sheet,    -   2) the positive electrode active material and the negative        electrode active material can be reversibly doped and dedoped        such that, where Qp (mAh) is an electric charge necessary for        causing total lithium contained in the positive electrode to be        dedoped and Qn (mAh) is an electric charge necessary for causing        lithium to fully dope the negative electrode, Qp>Qn, and    -   3) when the battery is charged at a charging current Ic (mA) in        a range of 0.2 Qn/h<Ic<2 Qn/h, in a range of an electric charge        for charging Qc (mAh) of 1<Qc/Qn<Qp/Qn, doping of the positive        electrode by lithium is started through lithium particles        produced on the negative electrode by charging of the battery        and is continued up to Qc>Qp.

The invention further provides a lithium ion secondary battery packcomprising the aforementioned lithium ion secondary battery and aheat-sensitive switch system such as a thermistor and/or PTC element.

The invention still further provides a method for charging theaforementioned lithium ion secondary battery, which method comprisescharging the lithium ion secondary battery by a constant current method,and determining completion of charging based on at least one of thefollowing: battery temperature increase, battery voltage drop or batteryvoltage oscillation.

The invention still further provides electric/electronic devicescomprising the aforementioned lithium ion secondary battery or lithiumion secondary battery pack.

The invention still further provides a lithium ion secondary batteryseparator comprising a sheet (A) with an average film thickness of 10-35μm, a basis weight of 6-20 g/m², a gas permeability (JIS P8117) of nolonger than 100 seconds, a MacMullin number of 10 or smaller and aMacMullin number×average film thickness value of no greater than 200 μm.

The invention still further provides a lithium ion secondary batteryseparator comprising a porous film with an average film thickness of10-35 μm and a basis weight of 10-25 g/m², which contains a porousorganic polymer film (B) that surrounds the above-mentioned sheet (A)and is swellable with the electrolyte solution and is retainable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing voltage changes during overcharging forExample 8 and Comparative Example 6.

FIG. 2 is a graph showing discharge behavior after overcharging forExample 8.

FIG. 3 is an electron micrograph showing the shape of lithium particlesproduced on a negative electrode during overcharging.

FIG. 4 is a graph showing the change in voltage and change in batterysurface temperature during overcharging for Example 12.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be explained in greater detail.

The lithium ion secondary battery of the invention is a batterycomprising a positive electrode, a negative electrode, a separator and anon-aqueous electrolyte, wherein

-   -   1) the separator is composed essentially of a porous sheet,    -   2) the positive electrode active material and the negative        electrode active material can be reversibly doped and dedoped        such that, where Qp (mAh) is an electric charge necessary for        causing total lithium contained in the positive electrode to be        dedoped and Qn (mAh) is an electric charge necessary for causing        lithium to fully dope the negative electrode, Qp>Qn, and    -   3) when the battery is charged at a charging current Ic (mA) in        a range of 0.2 Qn/h<Ic<2 Qn/h, in a range of an electric charge        for charging Qc (mAh) of 1<Qc/Qn<Qp/Qn, doping of the positive        electrode by lithium is started through lithium particles        produced on the negative electrode by charging of the battery        and is continued up to Qc>Qp, and it exhibits the        characteristics described below.        [Overcharge-Preventing Function]

When charging the lithium ion secondary battery of the invention at apractical charging current Ic such that 0.2 Qn/h<Ic<2 Qn/h, dopingcannot occur at the negative electrode during overcharge, such that thedeposited lithium particles reach the vicinity of the positive electrodesurface, doping the positive electrode, and this phenomenon continues upto Qc>Qp, thus preventing any further overcharge of the battery. Thepresent inventors discovered that, if lithium particles deposited on thenegative electrode surface are allowed to reach the vicinity of thepositive electrode surface to a shallow overcharge depth at which theelectric charge Qc is such that 1<Qc/Qn<Qp/Qn, then it is not onlypossible to guarantee the safety of the lithium ion secondary batteryduring overcharge, but the lithium particles reaching the vicinity ofthe positive electrode surface are doped at the positive electrode sothat complete internal short-circuit does not occur between the positiveand negative electrodes, and discharge is possible even afterovercharge.

This overcharge-preventing function is characterized in thatovercharging is prevented by doping of the positive electrode withlithium particles produced on the negative electrode duringovercharging, and the safety of the battery is guaranteed duringovercharging by continuation of this phenomenon until Qc>Qp; however, inorder to guarantee even higher safety of the battery, the phenomenon ispreferably continued up to Qc/3 Qn.

A lithium ion secondary battery in which this phenomenon occurs withQc/Qn<1 is a battery that cannot be fully charged, and is therefore notpreferred. When the phenomenon does not appear with Qc/Qn<Qp/Qn, thephenomenon cannot occur with Qc/Qn>Qp/Qn, for reasons related to thelithium amount.

Needless to mention, in order to obtain a lithium ion secondary batterywith this overcharge-preventing function, it is necessary for Qp and Qnto be such that Qp>Qn. The overcharge-preventing function of the batteryof the invention is achieved by utilizing the lithium particles beingdeposited at the negative electrode surface during overcharging, butthese lithium particles were originally contained in the positiveelectrode, and since the electric charge amount that can be used at thattime is Qp−Qn, the overcharge-preventing function of the invention isimpossible, in principle, if Qp<Qn.

In consideration of higher safety for the lithium ion secondary batteryof the invention, it is preferred to use half of the amount of lithiumremaining at the positive electrode when the negative electrode has beencompletely doped with lithium (Qp−Qn), to exhibit theovercharge-preventing function. That is, it is more preferred to exhibitthe overcharge-preventing effect with a more shallow charging depth, atwhich Qc is such that 1<Qc/Qn<0.5(Qp/Qn+1).

Here, Qp and Qn may be calculated from the weight of the positiveelectrode active material and negative electrode active material perunit area. They may also be determined by forming a three-electrodesystem battery using the positive electrode or negative electrode as theworking electrode and lithium metal as the reference electrode andcounter electrode, and measuring the charge/discharge.

The following method may be used to confirm this overcharge-preventingfunction in the battery of the invention. Specifically, the positiveelectrode and negative electrode are removed from the lithium ionsecondary battery of the invention, and Qp and Qn are measured by themethod described above to confirm the relationship Qp>Qn. Also, thepositive electrode, negative electrode and separator are removed fromthe lithium ion secondary battery of the invention, the positiveelectrode and negative electrode are connected via the separator, and anelectrolyte (electrolyte solution) is injected to fabricate evaluationbattery 1. It is confirmed that when evaluation battery 1 is chargedwith a charging current Ic satisfying the condition 0.2 Qn/h<Ic<2 Qn/h,there is observed a battery voltage drop, a battery voltage oscillationor a general halt in battery voltage increase when Qc is in the range of1<Qc/Qn<Qp/Qn. The electric charge is defined as Q1 (mAh) uponinitiation of battery voltage drop, a battery voltage oscillation or ageneral halt in battery voltage increase. It is also confirmed that thebattery voltage oscillation or the general halt in battery voltageincrease, or depending on the case the battery voltage drop, continuesup to Qc>Qp. The separator and positive electrode are then removed fromthe lithium ion secondary battery of the invention, and the positiveelectrode is connected to a negative electrode collector (i.e., Qn=0)via the separator, and an electrolyte (electrolyte solution) is injectedto fabricate evaluation battery 2. For charging at a charging current Icsatisfying 0.2 Qn/h<Ic<2 Qn/h, with the negative electrode collector inthis evaluation battery 2 as the negative electrode, the electric chargeis defined as Q2 (mAh) upon initiation of battery voltage drop, batteryvoltage oscillation or general halt in battery voltage increase. Here,if the relationship Q1>Q2 (ideally Q2=Q1−Qn) is true, then the observedbattery voltage drop, battery voltage oscillation or general halt inbattery voltage increase will depend on the lithium particles producedon the negative electrode which were originally contained in thepositive electrode, and the desired overcharge-preventing function canbe confirmed.

In the test described above, when removal of the negative electrodecollector is difficult, a copper foil or lithium foil may be usedinstead of the electrode collector. An electrolyte solution commonlyused in lithium ion secondary batteries may be employed as theelectrolyte solution for the test.

When the overcharge-preventing function is exhibited in a lithium ionsecondary battery of the invention, the electrical energy introduced bythe procedure of charging is released out of the system as Joule's heat.The Joule's heat is represented by (charging current×battery voltage).Thus, if a large charging current is used the effect of Joule's heatwill be dominant, and it will be difficult to accurately evaluate theovercharge-preventing effect. For application in a practical chargingcurrent Ic range of 0.2 Qn/h<Ic<2 Qn/h, a smaller capacity is preferredfor the evaluation battery. A button-type (coin-type) battery ispreferable for the evaluation batteries described above, but there is nolimitation thereto.

The aforementioned overcharge-preventing function may be explained asfollows for the most common type of lithium ion secondary batteryemploying lithium cobaltate as the positive electrode and a carbonmaterial that can be doped/dedoped with lithium as the negativeelectrode. This type of lithium ion secondary battery system is usuallydesigned so that Qn is about half of the Qp. That is, 2 Qn=Qp. Withconventional lithium ion secondary batteries of this type at sizes usedfor cellular phones, explosion and combustion occur if the charging rateat which all of the lithium is depleted from the positive electrode whencharging at 1C (charging current Ic=Qn/h) exceeds approximately 200% ofcharging percentage (electric charge Qc=Qp).

In light of this, it is possible to guarantee safety during overchargeby preventing total depletion of the lithium from the positiveelectrode, i.e. by halting further charging, but theovercharge-preventing function in the lithium ion secondary battery ofthe invention is characterized by accomplishing this via the lithiumparticles deposited on the negative electrode during overcharge. Thatis, it is necessary to start doping of the positive electrode withlithium particles at a charging percentage of no greater than 200%(Qc<Qp), and more preferably no greater than 150% (Qc<0.5(Qp+Qn)).

Overcharging will not be prevented without continued doping of thepositive electrode with the lithium particles deposited on the negativeelectrode. That is, it must be continued at a charging rate of at least200% (Qc>Qp) and more preferably at least 300% (Qc>3 Qn).

The lithium ion secondary battery of the invention having anovercharge-preventing function based on the mechanism described abovehas the following features.

1) When the battery is charged at a charging current Ic which satisfies0.2 Qn/h<Ic<2 Qn/h, Qc at which a drop in battery voltage occurs, anoscillation in battery voltage occurs, or the increase in batteryvoltage generally halts is in the range of 1<Qc/Qn<Qp/Qn.

Here, a drop in battery voltage occurs when the lithium particlesproduced on the negative electrode are doped on the positive electrode.An oscillation in battery voltage indicates intermittent doping of thelithium particles. The battery voltage drop and oscillation depend onthe internal resistance of the battery, being more difficult to observewhen the internal resistance of the battery is lower. In such cases,they can sometimes be observed by radically reducing the voltagesampling time. The battery voltage drop and battery voltage oscillationare also very difficult to observe in cases with a very rapid cycle ofdeposition of the lithium particles on the negative electrode and dopingonto the positive electrode. When the turn over cycle is rapid and theinternal impedance of the battery is low, an apparent phenomenon of ageneral halt in battery voltage increase is observed.

2) When the battery is charged at a charging current Ic which satisfies0.2 Qn/h<Ic<2 Qn/h, battery voltage oscillation of 10 mV or greaterstarts and continues to Qc>Qp, when the electric charge Qc is in therange of 1<Qc/Qn<Qp/Qn.

Thus, the battery voltage oscillation is proof of repetition of thecycle of deposition of lithium particles on the negative electrode anddoping onto the positive electrode, and an adequateovercharge-preventing function is exhibited if this continues up toQc<Qp.

3) When the battery is charged at a charging current Ic which satisfies0.2 Qn/h<Ic<2 Qn/h to Qc such that 2<Qc/Qn<3 and then discharged at adischarging current Id such that 0.1 Qn/h<Id<0.5 Qn/h, the dischargecapacity Qd is in the range of 1<Qd/Qn<Qp/Qn.

The lithium ion secondary battery of the invention can be dischargedthereafter, since a complete internal short-circuit does not occur. Ifthe overcharge-preventing mechanism is working, then the dischargecapacity Qd is in the range of 1<Qd/Qn<Qp/Qn, taking into account thelithium remaining on the negative electrode.

4) When charging with a charging current Ic satisfying 0.2 Qn/h<Ic<2Qn/h, the battery voltage is no greater than 5.5 V in the total rangewhere the electric charge Qc is such that 1<Qc/Qn<1.5 Qp/Qn.

Since further charging is prevented in the lithium ion secondary batteryof the invention by the mechanism described above, the battery voltagenever rises to a voltage at which the electrolyte (electrolyte solution)decomposes. This means that when the overcharge-preventing function isworking, the voltage never exceeds 5.5 V so long as a positive electrodematerial is used which is currently in general use.

5) The relationship between the battery internal impedance R_(0.5) at 1kHz when Qc satisfies Qc/Qn=0.5 and the battery internal impedance R₂ at1 kHz when Qc=Qp, is 1.5 R_(0.5)>R₂:

When the overcharge-preventing function is working, the decompositionwhich usually accompanies overcharge does not occur, and hence there isno notable increase in the internal impedance of the battery.

6) For Qc in the range of Qp<Qc<1.5 Qp, the maximum length of thelithium particles produced on the negative electrode by charging of thebattery is 100 μm or smaller.

This overcharge-preventing function is exhibited because the lithiumparticles deposited on the negative electrode are rendered fine so thatthey rapidly reach the vicinity of the positive electrode surface.

The lithium ion secondary battery of the invention possesses all of theabove features 1) to 6) in principle, but when the charging currentincreases with greater battery capacity, there will be some cases inwhich not all of the features are exhibited due to the problem ofJoule's heat. Generally speaking, the overcharge-preventing functionrequired for the lithium ion secondary battery of the invention can beconfirmed by satisfying one, or more preferably two of the abovefeatures 1) to 6).

As an expected result of the overcharge-preventing function, the lithiumion secondary battery of the invention produces no abnormaldecomposition of the electrolyte solution during overcharge, but alsocauses no destruction of the crystal structure of the positiveelectrode. The absence of abnormal decomposition of the electrolytesolution can be confirmed by a method such as GC-MS. The absence ofdestruction of the crystal structure of the positive electrode can beconfirmed from the peak pattern in X-ray diffraction.

[Electrodes]

For the positive electrode and negative electrode in the lithium ionsecondary battery of the invention there are used active materialscapable of reversible doping and dedoping of lithium, and materialscommonly employed in lithium ion secondary batteries may be used withoutany particular restrictions so long as Qp and Qn satisfy therelationship Qp<Qn.

The positive electrode and negative electrode will generally each becomposed of an active material, a binder polymer that binds the activematerial and retains the electrolyte solution, and a collector. Aconducting aid may also be added to improve the conductivity of theelectrodes.

A lithium-containing transition metal oxide such as LiCoO₂, LiMn₂O₄ orLiNiO₂ is preferably used as the positive electrode active material inthe lithium ion secondary battery of the invention. For the negativeelectrode active material there is preferably used a sintered organicpolymer compound such as polyacrylonitrile, phenol resin, phenol-novolacresin or cellulose, sintered coke or pitch, or a carbon material such asartificial graphite or natural graphite.

As binder polymers there may be used fluorine resins such aspolyvinylidene fluoride (PVdF), PVdF copolymer resins includingcopolymers of PVdF and hexafluoropropylene (HFP) or perfluoromethylvinyl ether (PFMV) and tetrafluoroethylene, and polytetrafluoroethylene,fluorine rubber, etc.; hydrocarbon-based polymers such asstyrene-butadiene copolymer and styrene-acrylonitrile copolymer; andcarboxymethyl cellulose, polyimide resins, and the like, but there is nolimitation to these. They may also be used either alone or incombinations of two or more.

For the collectors, a material with excellent oxidation resistance isused at the positive electrode and a material with excellent reductionresistance is used at the negative electrode. Specifically, aluminum,stainless steel, etc. may be mentioned for the positive electrodecollector and copper, nickel, stainless steel, etc. may be mentioned forthe negative electrode collector. The shape may be that of a foil ormesh. Particularly, an aluminum foil is preferred for the positiveelectrode collector and a copper foil is preferred for the negativeelectrode collector.

Carbon black (acetylene black) is preferably used as a conducting aid,but there is no limitation thereto.

Regarding the mixing ratio of the active material, binder polymer andconducting aid, a range of 3-30 parts by weight is preferred for thebinder polymer and a range of 0-10 parts by weight is preferred for theconducting aid, with respect to 100 parts by weight of the activematerial.

The method of manufacturing the electrode is not particularlyrestricted, and any publicly known method may be employed.

[Separator]

Two forms of the separator are preferred for use in the lithium ionsecondary battery of the invention.

The first form is a sheet (A) with an average film thickness of 10-35μm, a basis weight of 6-20 g/m², a gas permeability (JIS P8117: the timerequired for 100 cc of air to permeate an area of 1 in² at a pressure of2.3 cmHg) of no longer than 100 seconds, a MacMullin number of 10 orsmaller and a MacMullin number×average film thickness value of nogreater than 200 μm. This type of sheet has large gaps and, as theassembled battery has numerous throughholes with an approximate ratio(curvature) of 1 between the minimum length between the positive andnegative electrodes measured along the throughholes in the separator andthe separator film thickness, it is easier for the lithium particlesproduced on the negative electrode to reach the vicinity of the positiveelectrode surface during overcharging. This is therefore preferred, inorder to exhibit the overcharge-preventing function described above.

A sheet with an average film thickness of less than 10 μm and a basisweight of less than 6 g/m² more readily exhibits theovercharge-preventing function, but it has insufficient strength and isprone to short-circuiting, and is therefore not preferred as theseparator.

The normalized puncture strength can serve as an indicator inconsideration of avoiding short-circuiting. The normalized puncturestrength is preferably at least 3 g/μm, and the effective puncturestrength (normalized puncture strength×film thickness) is preferably atleast 80 g. The normalized puncture strength is the value determined bysetting the sheet in a 11.3 mmφ fixed frame, sticking a needle with a0.5 mm tip radius vertically into the center of the sheet, pushing theneedle in at a fixed speed of 50 mm/min, and normalizing the force onthe needle when a hole is opened in the sheet based on the average filmthickness of the sheet.

With a sheet having an average film thickness of less than 10 μm and abasis weight of less than 6 g/m², it is difficult to obtain a sheetsatisfying the conditions for the puncture strength.

Also, a sheet having an average film thickness of greater than 35 μm, abasis weight of greater than 20 g/m² and a gas permeability (JIS P8117)exceeding 100 seconds is not only disadvantageous in terms of theovercharge-preventing function, but is also not preferred from thestandpoint of reduced battery characteristics and reduced energy densityaccompanying the increased internal impedance. When the batterycharacteristics are considered, the sheet (A) preferably has a MacMullinnumber of 10 or smaller and a MacMullin number×average film thicknessvalue of no greater than 200 μm. More preferably, the MacMullinnumber×average film thickness value is no greater than 150 μm. Here, theMacMullin number is an indicator of the ion conductivity of the batteryseparator, and it is the ratio of the impedance of the sheet (A)impregnated with the electrolyte solution with respect to the impedanceof the electrolyte solution alone. Throughout the present specification,the MacMullin number refers to the value measured at 25° C. When thesheet has an average film thickness of greater than 35 μm, a basisweight of greater than 20 g/m² and a gas permeability (JIS P8117)exceeding 100 seconds, it is difficult to satisfy the conditions for theMacMullin number and MacMullin number×average film thickness value.

As a specific examples for the sheet (A) there may be mentioned a sheethaving hard protrusions or laser-opened holes in a film with a structuresuch as a polyolefin fine porous film commonly used for separators inlithium ion secondary batteries, or a sheet formed from fibers such as anonwoven fabric; however, the sheet (A) is not particularly limited solong as it satisfies the conditions given above.

When the sheet (A) is composed of fibers, the average fiber diameter ofthe fibers composing the sheet is preferably ½ to {fraction (1/10)} ofthe average film thickness of the sheet (A). If the average fiberdiameter is less than {fraction (1/10)} of the average film thickness ofthe sheet (A), the curvature is increased, not only making it impossibleto obtain adequate battery characteristics but also posing adisadvantage in terms of the overcharge-preventing function. If theaverage fiber diameter is greater than ½ of the average film thicknessof the sheet (A), there is less intertwining between the fibers, makingit impossible to obtain a sheet with adequate strength. The mesh of thesheet (A) is also too large, causing short-circuiting problems when thebattery is fabricated. When such a sheet (A) is used with the secondform of the separator of the invention described below, numerouspinholes are produced and a satisfactory separator cannot be obtained.

When the sheet (A) is composed of fibers, it is preferably in the formof a nonwoven fabric. The method of fabricating the nonwoven fabric maybe any commonly used dry sheeting method, spun bond method, water needlemethod, spun lace method, wet sheeting method and melt blow method. Thewet sheeting method is particularly preferred among these methodsbecause it can easily give a uniform, thin nonwoven fabric.

The overcharge-preventing function of the lithium ion secondary batteryof the invention is believed to be closely related to the separatorstructure (texture), and not particularly related to the material ofwhich the sheet (A) is composed. In other words, the sheet (A) can besuitably used so long as it is made of a material with sufficientoxidation resistance and reduction resistance. Such materials includepolyesters, aromatic polyamides, polyphenylene sulfide, polyolefins andthe like. These may be used alone or in combinations of two or more. Thematerial composing the sheet (A) need only have a molecular weightsufficient to obtain a molded article, and in most cases a molecularweight (weight average molecular weight: Mw) of 5000 or greater issuitable.

The second form is a porous film with an average film thickness of 10-35μm and a basis weight of 10-25 g/m², which contains a porous organicpolymer film (B) that surrounds the above-mentioned sheet (A) and isswellable with the electrolyte solution and is retainable. Although thissecond form may be slightly inferior to the above-mentioned form inwhich the separator is composed of the sheet (A) alone in terms of theovercharge-preventing function since it may impede the lithium particlesdeposited on the negative electrode from reaching the vicinity of thepositive electrode surface during overcharge, it is still possible toamply guarantee the safety of the lithium ion secondary battery duringovercharge so long as the conditions described above are satisfied. Onthe other hand, the advantage of the second form over the first form isimproved retention of the electrolyte solution and greatershort-circuiting resistance, and it is therefore effective forfilm-cased batteries to meet the demand for strict conditions againstsolution leakage, or batteries having a structure in which the separatoris wrapped flat, such as in square-shaped batteries, in order to meetthe demand for strict conditions against short-circuiting.

If the average film thickness is less than 10 μm and the weight is lessthan 10 g/m² in this second form, sufficient strength as a separatorcannot be achieved, and problems such as short-circuiting can occur. Aswith the first form, it becomes difficult to achieve the characteristicvalues including a normalized puncture strength of 3 g/μm or greater andan effective puncture strength of 80 g or greater.

An average film thickness of greater than 35 μm and a basis weight ofgreater than 25 g/m² is not only disadvantageous for theovercharge-preventing function, but can also lead to reduced batterycharacteristics. In particular, the low temperature characteristics willsometimes be undesirable. As with the first form, it is difficult toobtain a porous film of this type having a MacMullin number of 10 orsmaller and a film thickness×MacMullin number value of no greater than200 μm.

As mentioned above, the overcharge-preventing function of the lithiumion secondary battery of the invention is essentially independent of thematerial of which the separator is composed, and therefore the porousorganic polymer film (B) need only be a material that can be swelledwith the electrolyte solution and retain it, and that has adequateoxidation/reduction-resistance for use in a battery. From thisstandpoint, suitable materials for the porous organic polymer film (B)include polyvinylidene fluoride (PVdF) copolymers composed primarily of(PVdF). The molecular weight of the PVdF copolymer is preferably in therange of 10,000 to 1 million as the weight average molecular weight(Mw).

A suitable polymerization ratio of the vinylidene fluoride (VdF) in thePVdF copolymer is in the range of 92-98 mole percent as the molarfraction of VdF. If the molar fraction of VdF exceeds 98%, thecrystallinity of the polymer is too high, not only making it difficultto form the separator but also undesirably lowering the degree ofswelling with the electrolyte solution. If the molar fraction of VdF isless than 92%, the crystallinity of the polymer is too low, and this mayundesirably reduce the mechanical properties and heat resistance of theporous film retaining the electrolyte solution.

As particularly preferred PVdF copolymers there may be mentionedterpolymers composed of VdF, HFP and CTFE. The copolymerizationcomposition of the copolymer is most preferably VdF/HFP(a)/CTFE(b)(where (a)=2-8 wt % and (b)=1-6 wt %].

If the copolymerization ratio of HFP (a) is less than 2 wt %, the degreeof swelling with non-aqueous electrolyte solutions tends to undesirablydecrease. If it exceeds 8 wt %, the elasticity of the film is reducedmaking it impossible to adequately retain a large amount of electrolytesolution, while the heat resistance also tends to undesirably decreasewhen the electrolyte solution is retained.

The copolymerization ratio of CTFE (b) is preferably 1-6 wt %. If theCTFE ratio is less than 1 wt % the effect of adding the CTFE isinadequate, and it tends to become difficult to maintain heat resistanceand improve the electrolyte solution retention. If it is added atgreater than 6 wt %, the degree of swelling with the electrolytesolution tends to be undesirably reduced.

These PVdF copolymers may be used alone, or they may be used as mixturesof two or more different copolymers. If necessary, they may be blendedwith electrolyte solution-swelling non-fluorine polymers such aspolyacrylonitrile (PAN), polymethyl methacrylate (PMMA) or polyethyleneoxide (PEO).

The porous film may also contain a porous inorganic filler if necessary,in addition to the sheet (A) and porous organic polymer film (B). Byincluding a porous inorganic filler it is possible to improve theshort-circuit resistance of the film without sacrificing ionconductivity. As suitable inorganic fillers there may be mentionedporous particles of silica, alumina or the like with a particle size of0.1-10 μm.

The porous film can be obtained by impregnating and coating the sheet(A) into a solution of the polymer used to form the porous organicpolymer film (B), and then removing the solvent. The following methodsmay be mentioned as specific methods for fabrication of the porous film.

1. A method in which a polymer used to form the porous organic polymerfilm (B), a solvent which dissolves it and is compatible with water, anda phase separating agent (gelling agent or pore forming agent) are mixedand dissolved together, the sheet (A) is impregnated and coated with thedope solution, and the resulting film is then immersed in an aqueouscoagulation bath to coagulate the polymer used to form the porousorganic polymer film (B), and finally washed and dried to obtain aporous film.

2. A method in which a polymer used to form the porous organic polymerfilm (B), a volatile solvent which dissolves it and a plasticizing agentare mixed and dissolved together and the sheet (A) is impregnated andcoated with the dope solution and then dried to remove the volatilesolvent, after which the plasticizing agent is dissolved and thenextracted with a volatile solvent that does not dissolve the polymerused to form the porous organic polymer film (B), and drying is carriedout to obtain a porous film.

3. A method in which a polymer used to form the porous organic polymerfilm (B) is mixed with a plasticizing agent, the mixture is heated toplasticize and melt the polymer used to form the porous organic polymerfilm (B) and the sheet (A) is impregnated and coated with this dope,after which the film is cooled to hardness, the plasticizer is dissolvedand then extracted with a volatile solvent that does not dissolve thepolymer used to form the porous organic polymer film (B), and drying iscarried out to obtain a porous film.

The second form of the separator of the invention is advantageous overthe first form in terms of the electrolyte solution impregnation andretention. The electrolyte solution impregnation can be evaluated by theamount of electrolyte solution impregnated. Throughout the presentspecification, the amount of electrolyte solution impregnation will berepresented by the weight percentage of electrolyte solution impregnatedwith respect to the dry weight of the separator. The retention of theseparator for the electrolyte solution can be evaluated by,specifically, centrifuging the separator impregnated with thenon-aqueous electrolyte solution with centrifugal force of 1400×g(gravitational acceleration) for 20 minutes to remove the weaklyretained non-aqueous electrolyte solution. Throughout the presentspecification, the retention of the electrolyte solution will berepresented by the weight percentage of the separator aftercentrifugation with respect to the separator before centrifugation. Theretention of the electrolyte solution is preferably at least 70 wt %,and more preferably at least 80 wt %.

Considering the handling property during manufacture of the battery, theseparator described above which is used in the lithium ion secondarybattery of the invention preferably has a proof stress of at least1.5×10² N/m, and especially at least 3.0×10² N/m. The proof stressindicates the elastic limit strength and the degree of tensile force atwhich the film can be handled, with a higher value signifying easierhandling and higher productivity.

The proof stress is usually determined by a tensile test. According tothe invention, the proof stress was calculated from a stress-straincurve obtained by cutting out a 1 cm×3 cm strip from the separator, andperforming a tensile test by tension at a pull rate of 20 mm/min.

The heat deformation temperature of the separator of the invention ispreferably 150° C. or higher, and more preferably 170° C. or higher. Theheat deformation temperature can be evaluated by thermal mechanicalanalysis (TMA). The evaluation of the heat deformation temperature byTMA can be determined by cutting a 4 mm wide strip from the separator,placing a 0.01 N load thereon, and determining the heat deformationtemperature as the temperature at which at least a 2% change in length(elongation) occurs when the temperature is increased at a rate of 10°C./min.

[Non-Aqueous Electrolyte]

The non-aqueous electrolyte used in the lithium ion secondary battery ofthe invention may be a solution of a lithium salt in a non-aqueoussolvent, as is commonly employed for lithium ion secondary batteries.

As specific examples of non-aqueous solvents there may be mentionedpropylene carbonate (PC), ethylene carbonate (EC), butylene carbonate(BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethylcarbonate (DEC), methylethyl carbonate (MEC), 1,2-dimethoxyethane (DME),1,2-diethoxyethane (DEE), γ-butyrolactone (γ-BL), sulfolane andacetonitrile. These non-aqueous solvents may be used alone or incombinations of two or more. Particularly suitable for use are any oneor more solvents selected from among PC, EC, γ-BL, DMC, DEC, MEC andDME.

As lithium salts that dissolve in such non-aqueous solvents there may bementioned lithium perchlorate (LiClO₄), lithium hexafluorophosphate(LiPF₆), lithium borotetrafluoride (LiBF₄), lithium hexafluoroarsenate(LiAsF₆), lithium trifluorosulfonate (CF₃ _(SO) ₃Li), lithiumperfluoromethylsulfonylimide [LiN(CF₃SO₂)₂] and lithiumperfluoroethylsulfonylimide [LiN(C₂F₅SO₂)₂], but there is no limitationto these. These may also be used in combinations of two or more. Theconcentration of the dissolved lithium salt is preferably in the rangeof 0.2-2 M (moles/L).

[Fabrication of Lithium Ion Secondary Battery]

The method of fabricating the lithium ion secondary battery of theinvention is not particularly restricted, and any publicly knownfabrication method for lithium ion secondary batteries may be employed.

Specifically, one common method involves placing the positive electrodeand negative electrode, which have been coupled via the separator, in acase and then injecting the electrolyte therein and sealing it. Vacuuminjection is preferably used as the method of injecting the electrolyte,but there is no particular limitation thereto. The coupled electrodesmay also be impregnated with the electrolyte solution before beingplaced in the case.

For film-cased batteries in which the case is a pack composed of analuminum-plastic laminated film, the electrodes and separator arepreferably integrated by bonding. In this case, the separator ispreferably a porous film of the aforementioned second form. Bonding ofthe separator and electrodes is accomplished mainly bythermo-compression bonding, and this may be carried out in a dry statewithout the electrolyte or in a wet state containing the electrolyte.When the adhesive strength between the separator and electrode issatisfactory, it is possible to fabricate the battery without athermo-compression bonding step.

[Lithium Ion Secondary Battery]

There are no particular restrictions on the shape of the lithium ionsecondary battery of the invention, and it may have any shape, such ascylindrical, prismatic, flat or button-shaped.

The case may be a steel can, aluminum can or a pack made of analuminum-plastic laminated film, but there is no limitation to these.

When the aforementioned overcharge-preventing function is exhibited withthe lithium ion secondary battery of the invention, the procedure ofcharging causes the introduced electrical energy to be discharged out ofthe system as Joule's heat. This Joule's heat is expressed as (chargingcurrent×battery voltage). Since the battery voltage is roughly constantwhile the overcharge-preventing function is exhibited, Ic is thedominant factor for the Joule's heat. Although it depends on the Icemployed, an increase in the internal battery temperature by the Joule'sheat can expose the battery to danger such as ignition. The shape of thelithium ion secondary battery of the invention is therefore selected toresult in satisfactory heat release efficiency, depending on the Icemployed. From the standpoint of improving the heat release efficiency,a film-cased battery using an aluminum-plastic laminated pack as thesheath is preferred. There may also be employed a method in which aradiator plate is mounted on the battery.

Generation of gas due to oxidative decomposition of the electrolytesolution is suppressed in the lithium ion secondary battery of theinvention as a result of the above-mentioned overcharge-preventingfunction. Strict conditions are required for film-cased batteries interms of battery swelling, and from this standpoint as well, the lithiumion secondary battery of the invention is preferably used in the form ofa film-cased battery. Also, although safety vents are commonly mountedin conventional lithium ion secondary batteries, since gas generationduring overcharge is suppressed in the lithium ion secondary battery ofthe invention, it is possible to sufficiently guarantee safety duringovercharge without provision of a safety vent. However, safety will ofcourse be further improved if a safety vent is present.

Safety during overcharge in the lithium ion secondary battery of theinvention may not be guaranteed in some cases depending on the chargingcurrent Ic employed, because of the problem of Joule's heat. Theovercharge-preventing function of the lithium ion secondary battery ofthe invention is not preferred in principle for high-current (rate)charging. However, even using the publicly known additives proposed forimprovement of safety during overcharge is inadequate for guaranteeingsafety during overcharge with high-current (rate) charging. Since theovercharge-preventing function of the lithium ion secondary battery ofthe invention is based on a fundamentally different mechanism than theuse of publicly known additives, such additives may also be used. Thecombined use of such publicly known additives can therefore adequatelyguarantee safety during overcharge even with high-current (rate)charging.

[Battery Pack]

The battery pack of the invention is provided with at least a lithiumion secondary battery of the invention and a heat-sensitive sensor(thermo-sensor) or heat-sensitive switch (thermistor and/or PTC).

As mentioned above, Joule's heat is generated when theovercharge-preventing function of the lithium ion secondary battery ofthe invention is exhibited. Overcharge can be detected in the lithiumion secondary battery of the invention by this heat energy, andtherefore provision of a heat-sensitive sensor (thermo-sensor) orheat-sensitive switch (thermistor and/or PTC) in the battery pack iseffective from the standpoint of safety.

Although safety during overcharge can be sufficiently guaranteed with aheat-sensitive sensor (thermo-sensor) or heat-sensitive switch(thermistor and/or PTC) in the lithium ion secondary battery of theinvention, it may also be equipped with a protection circuit. The safetyof the battery is further improved by provision of a protection circuit.

[Charging Method]

The charging method of the invention comprises charging the lithium ionsecondary battery of the invention by a constant current method, andjudging completion of charging by at least one of the following: batterytemperature increase, battery voltage drop or battery voltageoscillation.

The lithium ion secondary battery of the invention is characterized notonly by guaranteeing safety during overcharging by theovercharge-preventing function as described above, but also by beingcapable of discharge thereafter. The start of the overcharge-preventingfunction can be detected by the start of battery temperature increasedue to Joule's heat, battery voltage drop or battery voltageoscillation. Detection of the overcharge-preventing function means thatthe battery is fully charged. Thus, constant current charging ispossible whereby the completion of charging is judged by batterytemperature increase, battery voltage drop or battery voltageoscillation.

However, the charging method for the lithium ion secondary battery ofthe invention is not limited to the aforementioned method, and anycommonly employed constant current/constant voltage charging method maybe used.

[Electric/Electronic Devices]

According to the invention there are also provided electric/electronicdevices equipped with the lithium ion secondary battery or lithium ionsecondary battery pack of the invention.

The lithium ion secondary battery or lithium ion secondary battery packof the invention can be suitably used in portable electronic devicessuch as cellular phones or laptop computers. In particular, employingthe charging method described above can greatly shorten the chargingtime as compared to ordinary constant current/constant voltage chargingmethods.

The present invention will now be explained in greater detail by way ofexamples.

[Separator]

EXAMPLE 1

Binder polyethylene terephthalate (PET) short fibers with a fineness of0.22 dtex (average fiber diameter: approximately 4.5 μm) were blendedwith oriented and crystallized PET short fibers with a fineness of 0.33dtex (average fiber diameter: approximately 5.5 μm) at a weight ratio of4/6, and a film was formed to a basis weight of 12 g/m² by a wetsheeting method and subjected to calender rolling at 200° C., to obtaina nonwoven fabric sheet. The properties of the obtained sheet were asfollows.

Average film thickness: 18 μm, gas permeability: 0.07 seconds,normalized puncture strength: 5.0 g/μm (90 g), MacMullin number: 5.0(MacMullin number×film thickness value=90 μm).

EXAMPLE 2

Numerous holes with a diameter of 2 μm were opened evenly in apolypropylene (PP) fine porous film (CELGARD TM2400, product of CelgardCo.) using a needle. The properties of the obtained sheet were asfollows.

Average film thickness: 25 μm, basis weight: 13.5 g/m², gaspermeability: 80 seconds, normalized puncture strength: 12 g/μm (300 g),MacMullin number: 5.8 (MacMullin number×film thickness value=145 μm).

EXAMPLE 3

Binder PET short fibers with a fineness of 1.22 dtex (average fiberdiameter: approximately 11 μm) were blended with oriented andcrystallized PET short fibers with a fineness of 0.55 dtex (averagefiber diameter: approximately 7 μm) at a weight ratio of 5/5, and a filmwas formed to a basis weight of 12 g/m² by a wet sheeting method andsubjected to calender rolling at 160° C., to obtain a nonwoven fabricsheet. The properties of the obtained sheet were as follows.

Average film thickness: 18 μm, gas permeability: 0.04 seconds,normalized puncture strength: 6.5 g/μm (117 g), MacMullin number: 9.0(MacMullin number×film thickness value=162 μm).

A PVdF copolymer composed of VdF:HFP:CTFE=95.5:2.3:2.2 (molar ratio) wasdissolved in a mixed solvent containing N,N-dimethylacetamide (DMAc) andpolypropylene glycol (PPG) with an average molecular weight of 400 in aratio of 6/4 (weight ratio) at 60° C., to prepare a film-forming dopewith a copolymer concentration of 14 wt %. The obtained dope was usedfor impregnation and coating of the above-mentioned nonwoven fabricsheet, and then the resulting film was immersed in an aqueous solutionwith a solvent concentration of 40 wt %, coagulated and then washed anddried to obtain a porous film. The properties of the obtained porousfilm were as follows.

Average film thickness: 26 μm, basis weight: 21.1 g/m², normalizedpuncture strength: 5.5 g/μm (144 g), MacMullin number: 5.9 (MacMullinnumber×film thickness value=153 μm), proof stress: 3.5×10² N/m.

EXAMPLE 4

Crystallized m-aramide short fibers with a fineness of 0.9 dtex (fiberdiameter: approximately 10 μm) were used to form a film with a basisweight of 15 g/m² by a dry sheeting method, and this was subjected tocalender rolling at 320° C. to obtain a nonwoven fabric sheet. Theproperties of the obtained sheet were as follows.

Average film thickness: 30 μm, gas permeability: 0.04 seconds,normalized puncture strength: 5.6 g/μm (95 g), MacMullin number: 5.8(MacMullin number×film thickness value=98.6 μm).

A PVdF copolymer composed of VdF:HFP:CTFE=95.5:2.3:2.2 (molar ratio) wasdissolved in a mixed solvent containing N,N-dimethylacetamide (DMAC) andpolypropylene glycol (PPG) with an average molecular weight of 400 in aratio of 6/4 (weight ratio) at 60° C., to prepare a film-forming dopewith a copolymer concentration of 10 wt %. The obtained dope was usedfor impregnation and coating of the above-mentioned nonwoven fabricsheet, and then the resulting film was immersed in an aqueous solutionwith a solvent concentration of 40 wt %, coagulated and then washed anddried to obtain a porous film. The properties of the obtained porousfilm were as follows.

Average film thickness: 34 μm, basis weight: 20.9 g/m², normalizedpuncture strength: 9.7 g/μm (330 g), MacMullin number: 4.6 (MacMullinnumber×film thickness value=156 μm), proof stress: 6.4×10² N/m.

EXAMPLE 5

Binder PET short fibers with a fineness of 0.22 dtex (average fiberdiameter: approximately 4.5 μm) were blended with crystallized m-aramideshort fibers with a fineness of 0.55 dtex (average fiber diameter:approximately 7 μm) at a weight ratio of 4/6, and a film was formed to abasis weight of 11 g/m² by a wet sheeting method and subjected tocalender rolling at 200° C., to obtain a nonwoven fabric sheet. Theproperties of the obtained sheet were as follows.

Average film thickness: 17 μm, gas permeability: 0.06 seconds,normalized puncture strength: 5.6 g/μm (95 g), MacMullin number: 5.8(MacMullin number×film thickness value=99 μm).

After impregnating and coating this nonwoven fabric sheet with the samedope as prepared in Example 3, the obtained film was immersed in anaqueous solution with a solvent concentration of 40 wt %, coagulated andthen washed and dried to obtain a porous film. The properties of theobtained porous film were as follows.

Average film thickness: 24 μm, basis weight: 16.7 g/m², normalizedpuncture strength: 5.0 g/μm (120 g), MacMullin number: 5.4 (MacMullinnumber×film thickness value=130 μm), proof stress: 3.5×10² N/m.

EXAMPLE 6

A PVdF copolymer composed of VdF:HFP:CTFE=95.5:2.3:2.2 (molar ratio) wasdissolved in a mixed solvent containing N,N-dimethylacetamide (DMAC) andpolypropylene glycol (PPG) with an average molecular weight of 400 in aratio of 6.5/3.5 (weight ratio) at 60° C., to prepare a film-formingdope with a copolymer concentration of 12 wt %. The obtained dope wasused for impregnation and coating of the nonwoven fabric sheet formed inExample 1, and then the resulting film was immersed in an aqueoussolution with a solvent concentration of 40 wt %, coagulated and thenwashed and dried to obtain a porous film. The properties of the obtainedporous film were as follows.

Average film thickness: 24 μm, basis weight: 19.7 g/m², normalizedpuncture strength: 6.3 g/μm (151 g), MacMullin number: 6.5 (MacMullinnumber×film thickness value=156 μm), proof stress: 3.8×10² N/m.

EXAMPLE 7

Binder PET short fibers with a fineness of 1.22 dtex (average fiberdiameter: approximately 11 μm) were blended with oriented andcrystallized PET short fibers with a fineness of 0.11 dtex (averagefiber diameter: approximately 3.5 μm) at a weight ratio of 4/6, and afilm was formed to a basis weight of 12 g/m² by a wet sheeting methodand subjected to calender rolling at 130° C., to obtain a nonwovenfabric sheet. The properties of the obtained sheet were as follows.

Average film thickness: 14 μm, gas permeability: 0.60 seconds,normalized puncture strength: 8.9 g/μm (124 g), MacMullin number: 5.0(MacMullin number×film thickness value=70 μm).

After impregnating and coating this nonwoven fabric sheet with the samedope as prepared in Example 3, the obtained film was immersed in anaqueous solution with a solvent concentration of 40 wt %, coagulated andthen washed and dried to obtain a porous film. The properties of theobtained porous film were as follows.

Average film thickness: 24 μm, basis weight: 18.8 g/m², normalizedpuncture strength: 6.8 g/μm (164 g), MacMullin number: 4.9 (MacMullinnumber×film thickness value=118 μm), proof stress: 3.3×10² N/m.

The heat deformation temperature, etc. of the separators of Examples 1to 7 were measured, giving the results shown in Table 1 below. TABLE 1Heat deformation Gas temperature Impregnation Retention permeabilityExample 1 240° C. Example 2 155° C. Example 3 205° C. 48 wt % 83 wt % 22sec Example 4 300° C. 45 wt % 78 wt % 25 sec Example 5 280° C. 47 wt %82 wt % 20 sec Example 6 200° C. 50 wt % 85 wt % 30 sec Example 7 195°C. 52 wt % 88 wt % 22 sec

COMPARATIVE EXAMPLE 1

The separator used was a polypropylene (PP) fine porous film (CELGARDTM2400, product of Celgard Co.). The properties of the film were asfollows.

Average film thickness: 25 μm, basis weight: 14.8 g/m², gaspermeability: 350 seconds, normalized puncture strength: 15.2 g/μm (380g), MacMullin number: 6.5 (MacMullin number×film thickness value=163μm).

COMPARATIVE EXAMPLE 2

Crystallized m-aramide short fibers with a thickness of 0.9 dtex(average fiber diameter: approximately 10 μm) and m-aramide fibrit(synthetic pulp particles) were blended in a ratio of 8/2 (weightratio), and a film was formed to a basis weight of 30 g/m² by a wetsheeting method and subjected to calender rolling at 320° C., to obtaina paper-like sheet. The properties of the obtained sheet were asfollows.

Average film thickness: 35 μm, gas permeability: 38 seconds, normalizedpuncture strength: 16 g/μm (550 g), MacMullin number: 18.0 (MacMullinnumber×film thickness value=630 μm).

COMPARATIVE EXAMPLE 3

PET was used as the starting material to form a melt blown nonwovenfabric film with an average fiber diameter of 1.5 μm and a basis weightof 35 g/m². The nonwoven fabric was subjected to calender rolling at130° C. to form a 50 μm film. The properties of the nonwoven fabric wereas follows.

Gas permeability: 40 seconds, normalized puncture strength: 5.5 g/μm(275 g), MacMullin number: 3.8 (MacMullin number×film thicknessvalue=190 μm).

After impregnating and coating this nonwoven fabric sheet with the samedope as prepared in Example 3, the obtained film was immersed in anaqueous solution with a solvent concentration of 40 wt %, coagulated andthen washed and dried to obtain a porous film. The properties of theobtained porous film were as follows.

Average film thickness: 60 μm, basis weight: 43.5 g/m², normalizedpuncture strength: 60 g/μm (360 g), MacMullin number: 3.3 (MacMullinnumber×film thickness value=198 μm).

[Evaluation by Button (Coin) Battery]

EXAMPLE 8

[Positive Electrode]

A positive electrode paste was prepared using an N-methylpyrrolidone(NMP) solution with 6 wt % PVdF, comprising 89.5 parts by weight oflithium cobaltate (LiCoO₂, product of Nippon Chemical Industrial Co.,Ltd.) powder, 4.5 parts by weight of acetylene black and 6 parts by dryweight of PVdF. The obtained paste was coated and dried onto a 20 μmthick aluminum foil and then pressed to obtain a 97 μm thick positiveelectrode.

The total lithium content Qp of the positive electrode as calculatedfrom the positive electrode weight was 5.4 mAh/cm².

[Negative Electrode]

A negative electrode paste was prepared using an NMP solution with 6 wt% PVdF, comprising 87 parts by weight of mesophase carbon microbeads(MCMB, product of Osaka Gas Chemical Co., Ltd.) powder as the negativeelectrode active material, 3 parts by weight of acetylene black and 10parts by dry weight of PVdF. The obtained paste was coated and driedonto an 18 μm thick copper foil and then pressed to obtain a 90 μm thicknegative electrode.

The amount of lithium with which the negative electrode could be dopedQn was 2.6 Ah/cm², as measured with a three electrode battery.

[Fabrication of Button (Coin) Batteries]

The above-mentioned positive electrode and negative electrode werepunched out into a 14 mm diameter circle, and the separators fabricatedin Examples 1 to 7 were used after punching out to a 16 mm diameter. Thepositive and negative electrodes were joined via each of the separators,impregnated with an electrolyte (electrolyte solution) and encapsulatedinto battery cases. The electrolyte solution used was 1 M LiPF₆ EC/DEC(1/1 weight ratio). The battery case size was CR2032.

[Overcharge Evaluation]

The overcharge evaluation of each of the fabricated button batteries wascarried out by measurement of one charge/discharge cycle underconditions with constant current/constant voltage charging for 8 hoursat a charging current density of 0.52 mA/cm², charging voltage upto 4.2V and constant current discharge at a discharging current density of0.52 MA/cm² and a 2.75 V cutoff, after which overcharging was carriedout under conditions with constant current charging for 10 hours at acharging current density of 2.6 MA/cm². After standing for 2 hoursfollowing the overcharging, discharge was carried out under conditionswith a discharging current density of 0.52 MA/cm² and a 2.75 V cutoff.The battery voltage sampling time was every 30 seconds.

Battery voltage oscillation was observed during charging with all of thebatteries, and the battery voltage never increased above 5.5 V duringcharging. Also, all of the batteries had an open circuit voltage in therange of 4.2-4.5 V after overcharging, and were thus capable ofdischarge after overcharging.

Table 2 shows the electric charge (Q1) at the start of battery voltageoscillation and the discharge capacity Qd obtained after overcharging.FIG. 1 shows the change in voltage during overcharging and FIG. 2 showsthe discharge behavior after overcharging, for a battery employing theseparator of Example 7, as one example.

COMPARATIVE EXAMPLE 4

Button batteries were fabricated using the separators of ComparativeExamples 1 to 3, by the same method as Example 8. The button batterieswere subjected to testing in the same manner as Example 8.

The battery employing the separator of Comparative Example 2 did notexhibit adequate properties in the first charge/discharge cycle, as aresult of the high resistance of the separator. Therefore, noovercharging test was conducted for the battery employing the separatorof Comparative Example 2.

The batteries employing the separators of Comparative Examples 1 and 3exhibited satisfactory first charge/discharge, and were thereforesubjected to an overcharging test. However, no battery voltageoscillation was observed, and the battery voltage increased above 5.5 Vmaking further discharge impossible. FIG. 1 shows the change in voltageduring overcharging for the battery employing the separator ofComparative Example 1.

These results demonstrate that the batteries employing the separators ofComparative Examples 1 and 3 clearly exhibit no overcharge-preventingfunctions as were exhibited by the lithium ion secondary batteries ofthe invention.

EXAMPLE 9

Button batteries were fabricated with the separators of Examples 1 to 7,by the same method as Example 8, but using a copper foil as the negativeelectrode. The button batteries were subjected to constant currentcharging at a charging current density of 0.56 mA/cm², with the copperfoil as the negative electrode. With all of the batteries there wasobserved a battery voltage drop, a battery voltage oscillation or ageneral halt in battery voltage increase.

Table 2 shows the electric charge Q2 at the start of this phenomenon.

EXAMPLE 10

Button batteries were fabricated with the separators of Examples 1 to 7,by the same method as Example 8. The button batteries were subjected toan overcharging test under the same conditions as Example 8. During theovercharging test, the impedance R_(0.5) at 1 kHz was measured when theQc reached 1.3 mAh/cm². Also, the impedance R2 at 1 kHz was measuredwhen the Qc reached 5.4 mAh/cm².

Table 2 shows the values for R₂/R_(0.5).

EXAMPLE 11

Button batteries were fabricated with the separators of Examples 1 to 7,by the same method as Example 8. The button batteries were subjected toan overcharging test under the same conditions as Example 8. When the Qcin the overcharging test reached 7 mAh/cm², overcharging was halted, thebattery was taken apart and the negative electrode surface was observedwith a scanning electron microscope (SEM). FIG. 3 shows a SEM photographof the separator used in Example 5, but the same interspersion oflithium particles with a maximum length of 100 μm was observed in all ofthe batteries.

With the separators of Examples 3 to 7, surface pores corresponding tothese lithium particles were observed on the side in contact with thenegative electrode, and no surface pores were observed on the side incontact with the positive electrode.

The results for Examples 8 to 11 demonstrate that the lithium ionsecondary batteries of the invention employing lithium ion secondarybattery separators according to the invention exhibit anovercharge-preventing function by the lithium particles, as explainedabove.

[Evaluation of Film-Cased Battery]

EXAMPLE 12

The positive electrode and negative electrode fabricated in Example 8were used, with equal areas, for fabrication of a film-sheathed batteryusing the separator of Example 5. The film-sheathed battery wasfabricated by connecting the positive electrode and negative electrodevia the separator, placing the combination in an aluminum-plasticlaminated pack, and injecting an electrolyte solution. The electrolytesolution used for this film-cased battery was prepared by dissolvingLiPF₆ to 1 M in a mixed solvent having a composition of EC:DEC:MEC=1:1:1(weight ratio). The size of the film-cased battery was 55 mm×35 mm×3.7mm.

Upon measurement of the initial charge/discharge in the same manner asExample 8, the film-cased battery was found to have a capacity of 650mAh.

The film-cased battery was overcharged by constant current charging to aQc of 1950 mAh (350% charging percentage) at a charging current densityof 2.6 mA/cm², at an environment temperature of 25° C. The batteryvoltage increase generally halted at a 120% charging percentage,leveling off at about 4.5-4.6 V. The battery surface temperature beganto rise simultaneously with the general halt in the battery voltageincrease, leveling off at about 50° C., with a maximum battery surfacetemperature of 51° C. (FIG. 4). No swelling of the film-cased batterywas observed during this overcharging. The charging percentage referredto here is the percentage of the time elapsed for the charging procedurewith respect to the time required for full charging.

After the overcharging, constant current discharge was carried out to2.75 V at a discharging current density of 0.52 mA/cm². A dischargecapacity of 656 mAh was obtained.

These results demonstrate that the overcharge-preventing function of thelithium ion secondary battery of the invention can be effectivelyachieved even with a practical battery size.

EXAMPLE 13

A film-cased battery having the same construction as the film-casedbattery fabricated in Example 12 was overcharged to a 186% chargingpercentage at a charging current density of 2.6 mA/cm², and the batterywas subjected to an oven heating test. The temperature elevating rate inthe oven was 5° C./min, and the battery was allowed to stand for onehour when the temperature reached 150° C. As a result, no rupture orfire occurred, despite swelling of the battery.

COMPARATIVE EXAMPLE 5

A film-cased battery was fabricated in the same manner as Example 12,except for using the separator of Comparative Example 1, and this wasovercharged to a 186% charging percentage at a charging current densityof 2.6 mA/cm², after which the battery was subjected to an oven heatingtest under the same conditions as Example 13. The battery ruptured andfired when the oven temperature reached 125° C.

EXAMPLE 14

A film-cased battery was fabricated having the same construction as thefilm-cased battery fabricated in Example 12. This film-cased battery wassubjected to 5 cycles of an overcharging cycle test of charging to a Qcof 1300 mAh with a charging current density of 2.6 mA/cm² and constantcurrent discharging to 2.75 V at 0.52 mA/cm². No swelling, rupture orfire of the battery occurred during the test, and a discharge capacityof 448 mAh was obtained even after the 5th cycle.

The results of Examples 13 and 14 and Comparative Example 5 demonstratethat the lithium ion secondary battery of the invention is highly safeagainst overcharging compared to conventional lithium ion secondarybatteries.

EXAMPLE 15

A film-cased battery was fabricated having the same construction as thefilm-cased battery fabricated in Example 12, and a chalk marker (thermolabel) for detection of heat generation was attached to the surface ofthe film-cased battery. The battery was subjected to overcharging to aQc of 900 mAh at a charging current of 2.6 mA/cm². The attached chalkmarker exhibited a color change.

COMPARATIVE EXAMPLE 6

A film-cased battery was fabricated having the same construction as thefilm-cased battery fabricated in Comparative Example 5, and a chalkmarker for detection of heat generation was attached to the surface ofthe film-cased battery. The battery was subjected to overcharging to anelectric charge of 900 mAh at a charging current density of 2.6 mA/cm².The attached chalk marker exhibited no color change.

When the overcharge-preventing function was exhibited in the battery ofExample 15, the electrical energy introduced into the battery by theprocedure of charging was discharged out of the system as Joule's heatwithout being stored in the battery, and therefore the chalk markerchanged color. In contrast, with the battery of Comparative Example 6,all of the electrical energy introduced into the battery by charging wasstored in the battery with no heat generation, and therefore the chalkmarker did not change color.

The results for Example 15 and Comparative Example 6 demonstrate thatthe lithium ion secondary battery of the invention has the advantage ofallowing effective use of thermistor-type overcharge protectioncircuits, which have not been possible with conventional lithium ionsecondary batteries. TABLE 2 Q1 Qd Q2 (mAh/cm²) (mAh/cm²) (mAh/cm²)R₂/R_(0.5) Example 1 2.69 2.68 0.01 1.03 Example 2 3.75 3.12 0.90 1.05Example 3 3.06 2.86 0.34 1.07 Example 4 2.89 3.17 0.25 1.05 Example 53.11 2.89 0.32 1.05 Example 6 3.25 2.90 0.50 1.05 Example 7 4.27 3.861.56 1.10Industrial Applicability

According to the present invention, it is possible to preventovercharging in a lithium ion secondary battery by doping the positiveelectrode with lithium from the lithium particles produced on thenegative electrode during overcharge, and thus provide a lithium ionsecondary battery which is safe during overcharge.

1-21. (canceled)
 22. A lithium ion secondary battery separatorcomprising a sheet (A) with an average film thickness of 10-35 μm, abasis weight of 6-20 g/m², a gas permeability (JIS P8117) of no longerthan 100 seconds, a MacMullin number of 10 or smaller at 25° C. and aMacMullin number×average film thickness value of no greater than 200 μm.23. A separator according to claim 22, wherein the sheet is composed offibers and the average diameter of the fibers composing the sheet is ½to {fraction (1/10)} of the average film thickness of the sheet (A). 24.A separator according to claim 23, wherein the sheet (A) is a nonwovenfabric.
 25. A separator according to claim 22, wherein the sheet (A) iscomposed of at least one of a polyester, an aromatic polyamide, apolyphenylene sulfide and a polyolefin.
 26. A lithium ion secondarybattery separator comprising a porous film with an average filmthickness of 10-35 μm and a basis weight of 10-25 g/m², which contains aporous organic polymer film (B) that surrounds a sheet (A) according toclaim 22, and is swelled with the electrolyte solution and retains it.27. A separator according to claim 26, wherein the porous organicpolymer film (B) is composed mainly of polyvinylidene fluoride (PVdF).28. A separator according to claim 27, wherein the porous organicpolymer film (B) is composed of a polyvinylidene fluoride copolymercontaining 92-98% mole percent of vinylidene fluoride (VdF).
 29. Aseparator according to claim 28, wherein the porous organic polymer film(B) is composed of a terpolymer of vinylidene fluoride,hexafluoropropylene (HFP) and chlorotrifluoroethylene (CTFE).
 30. Aseparator according to claim 29, wherein the copolymer composition ofthe terpolymer is VdF/HFP(a)/CTFE(b), where (a)=2-8 wt % and (b)=1-6 wt%.